Fixes documentation around rolling upgrades

Moves coordinator removal into a 3rd cleanup step, having it removed from the normal upgrade process
Adds a warning that ha.coordinators should not be set for the last instance to be upgraded
Updates backup documentation to mention backups are enabled by default
Fixes documentation string and default value for tx_manager_impl configuration setting

Sets the default value of tx_manager_impl to "native", removing the explicit mention in the description
Updates usage in IAGD to check for equality with default rather than null